In the context of an aircraft engine, a typical starter ring gear is a metallic circular device having gear teeth fitted around its periphery. To start the aircraft engine, a start motor drives the gear teeth which transfers torque from the starter motor to rotate the engine to begin the cycle.
One conventional aircraft engine design has a starter ring gear disposed at a front end of the engine near a propeller which is driven by the engine. The starter ring gear includes a single pulley which is configured to drive a belt thus providing drive for an accessory such as an alternator.
In this conventional aircraft engine design, a gear reduction assembly resides at a back end of the engine (i.e., the end opposite the front end). The gear reduction assembly turns at a rate that is half of the engine's crank speed. This assembly provides a low resolution output which electronic circuitry uses to determine crank shaft position and current engine speed.